Method of fracturing formations



June 30, 1964 l.. E. ELKlNs METHOD OF FRACTURING FORMATIONS Filed Feb. 20, 1959 n r r n a r a r FIG.

INVENTOR. L OYD E. ELKINS ATTORNEY United States Patent @ffice 3,139,139 Patented June 30, 1964 3,139,139 METHOD OF FRACTURING FORMATIONS Lloyd E. Elkins, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Feb. 29, 1959, Ser. No. 794,569 Claims. (Cl. 166-42) This invention relates to an improved method of horizontally fracturing formations. More particularly, this invention relates to a method of hydraulically fracturing subterranean formations wherein long horizontal fractures are selectively produced.

Permeable formations penetrated by a well have been fractured by the application of a high hydraulic pressure in the well as disclosed in Re. 23,733, reissued to Riley F. Farris on November 10, 1953. While that process, as previously disclosed and practiced, normally produces horizontal fractures, i.e., fractures along bedding planes intersected by the well, occasionally a vertical fracture, i.e., a fracture longitudinally of the well, is produced. Vertical fractures are in some instances desirable, but generally they are undesirable, particularly when they extend vertically into water or gas producing horizons adjacent a fractured oil producing formation. It has now been found that a horizontal fracture can to a large extent be selectively produced by minimizing the vertical forces on the formation to be fractured, i.e., by reducing to a minimum the effective overburden pressure on that formation, and that the depth or extent of such horizontal fractures can be increased by minimizing these vertical forces on the formation at great distances from a well.

It is an object of this invention to provide an improved method of fracturing formations penetrated by a well. It is a more specific object of this invention to provide a method of selectively producing long horizontal fractures in an elastic formation penetrated by a well. Other objects of this invention will be apparent from the following description in which:

FIGURE 1 is a cross-sectional View of a well showing apparatus and construction for 'carrying out the steps of this invention, and

FIGURE 2 is a cross-sectional view of the lower end of a well showing a modified construction for carrying out the steps of this invention.

This invention may be described in brief as an improved method for hydraulically fracturing subterranean elastic formations, such as sandstone or limestone, penetrated by wells wherein deeply penetrating horizontal fractures are selectively produced by substantially reducing over a large area surrounding such wells the effective overburden pressure on the formation to be fractured as the fracturing iluid is injected into that formation. This overburden pressure is reduced by producing a cavity or otherwise enlarging or undercutting a well in a zone which encompasses or spans a plastic formation such as shale that is adjacent to the elastic formation to be fractured. The cavity is vthen cased off or otherwise isolated from the remainder of the well to maintain a relatively low hydrostatic pressure is confined, the shale performs like an elastic rock, and that when not confined, the shale becomes plastic and flows. An important aspect of this invention is in the discovery that the area of plastic flow in a substantially horizontal plane is proportional to the cross-sectional area in the same plane of a hole or cavity in a shale bed. A well is therefore completed as shown in both FIGURES 1 and 2 in such manner that the shale or other plastic fonnation 12 has considerable room for flow so that it will be relaxed a great distance from the well. As the well is being drilled or at least during completion of the well, one or more cavities 14 are produced in plastic formations which have been located by drillers logs or the like and are adjacent formations to be fractured. These cavities which typically are produced by underreaming the well are generally therefore coaxial with the well and have an area from two to about ten times or more greater than the area of the well. An underreamed hole about four feet in diameter is typical. Since the height or thickness of the cavity appears to have'no effect upon the producing of fractures, it may be varied over a substantial range depending upon the method and apparatus used to produce the cavity. The cavity is generally produced with a mechanical underreamer ofthe expanding blade or cutter type. Washing or jetting with liquid or abrading with solids in a fluid are alternates. Such cavities, whether formed by these methods or by others, can be produced either while drilling the well or after the well is completed. Where the plastic formation is subjacent and preferably immediately below the formation to be fractured as shown in FIGUREQ, the'well may be drilled, a casing 15 run therein and set with a sheath of cement 16 in or through the elastic formation 13 before the cavity is formed below the lower end of the casing. In most cases a cavity is formed in a plastic formation superjacent and preferably immediately above the elastic formation to be fractured as shown in FIG- URE 1. Such cavities are, of course, formed prior to running and setting of the casing or a liner therein so that the cavity can be isolated fromv the well and particularly from the conduit such as the casing through which the fracturing fluid is injected into the well. YIn either case the casing may be set in the upper part of the producing zone, i.e., the elastic formation 13, using the open hole completion process or technique. With an open hole completion a bridging plug or formation packer (not shown) is in the embodiment ofFIGURE 2 set in the well below the casing to isolate the well and a subjacent cavity. Typically casing is set through the producing zone and a number of perforations 17 are placed therein opposite the particular zone or zones of the elastic formation 13 which are to be fracturedv horizontally. The uid in the casing is then inthe apparatus shown in FIGURE 1 isolated from the cavity by a section 18 of the cement sheath between the perforations and the cavity. In the apparatus shown in FIG- URE 2 the uid in the casing is isolated via the inside of the casing from the cavity by closing the end of the casing as by a bridging plug 19 set in the casing below the perforations 17. On the outside of the casing, in the latter case, the section 18 ofcement sheath below perforations 17 and the cementing basket 21 xed to the lower end of the casing prevent the fracturing fluids leaving the casing via the perforations from bypassing through the well directly to the cavity 14.

In operation, after the plastic formation 12 has been located by cuttings analysis, drillers log, electric log, or the like, and particularly after the shale-permeable formation interface 22 has been located, the cavity 14 is produced and the well apparatus, including a pump It has now been found that when a shale formation@ 23 connected to discharge into the casing, is installed ysubstantial distance radially into the formation.V

permeable path is provided.

as above described. When the cavity is produced, the

Y overburden pressure being greater than the pressure of the Huid in the well, thisvshaleV or. plastic formation 12,k at least 'partially (relaxes, i.e., flows toward. the well to decreasethe volume of the cavity, Vsimultaneously reducing the vertical stress in the plastic shale for "a In some cases after the cavity isA produced, liquid lisremoved from-the well to accelerate this iiow of the plastic formation into'the Well. This movement ofthe shale and theV resulting reduction of the vertical stress, ri.e.,v v,the effective` overburden pressure, thus either initiates a fracture at the interface 22. or, due to the reduction` ofV this effective overburden Vpressure at that interface, causes a fracture 241to` beinitiated at a relatively llow pressure when hydraulic pressure is subsequently applied to an adjacentrzone in the permeable elastic for-V mation 13. While I do notV wish tobe limitedv to any ing liquid through said well yinto said, isolated zone un` der sufficient pressure to fracturesaid elastic formation.

theory. of operation, it appears that in some cases the'U movement of the shale into the well produces shear at the interfaces, eg., interface 22, between a shale.

zone and an adjacent rock formation, the greater Ythe areak of the cavity th'egreater the,v area of shear,so that a fracture is not only initiated at the interface but `is extended deepv intothe strata surrounding the well.

After the Welland the cavity ory cavities havebeen I kcompleted andV sufficient time has elapsed, eg., a. day

or more, for the plastic formationv to approach Vequi- 23, casing 15, and perforations 17 into'the. permeable elasticv formation 13, rThis fracturing uid enters Vany horizontalfractures initiated bythe previousrelaxation -librium, a quantity of fracturing iluid is injectedsvia pump and movement of the shale or otherwise places the elasticformationv 13 under sufficient stress to produce or 'otherwise Yextend horizontal fracture 24 deep into elasticwformation 13. Formation props 25j carried'in the fracturmg uid "are therebyV placed in the fracture and remain there when the job is completed sothat when the Well is placed on production and iluid flows from the permeable elastic Vformation intothe Well, a highly From thel foregoing it canl beseen that this invenl a l and selectively injecting a fracturing fluid into said elastic formation through a vperforation in saidcasing .under suficient pressure to fracturey said elastic formation.

3. A method of producing a long horizontal fracture in a subterranean elastic formation which is adjacent` a plastic kformation comprising drilling a well intoboth said elastic formation and said plastic formation, underrreaming said well in said plastic formation to-produce a section 'of Venlarged diameter, isolating'a zone of said,` well within said elastic formation.l from iiuidl communication with said section of enlarged diameter, pausing Vfor ak periody ofV time 'suiiicient to permit relaxation` of vertical stresses in said plastic `formation rsaid zone being isolated by running a casing linto said well and into said elastic formation, perforating saidcasing ata level opposite said elastic formation,-andinjecting a fracturf4. A` method of producing Va long horizontal fracture inra/subterranean elastic'r formation which'is penetrated by a well and which is subjacen't Ya plasticV formation comprising undercutting said lplastic formation to produce a cavity therein, setting a casing in said well through said cavity to isolate fluid lin saidjcasing'from said lcavity, pausing fora yperiod of time sufficient to permit relaxation of vertical stresses in said plastic formation, perforating said 'casing atA a level opposite said elastic formation and then Jselectively injecting a fracturing`V fluid through said well into saidrelastic formation via the resulting perforations under suflicient *pressure to i fracture said elastic formation.

5L Amethod according to claim 4 wherein said plastic formation is undercutv radially from' said .well ,tov produce a cavity substantiallyv coaxial with but larger-than said well'. 1 Y

Q6. A method. according to claim 5 including pertorat ing said casing oppositesaid .elastic formation and injecting saidfracturing fluid into said plastic formation r. through a perforation.

tionis susceptible of a vwide/variety of embodiments and should, therefore, be construed not to lbe limited bythe above description but should be construed to he limited only by the' scope of ythe appended claims. Y j

Iclaim: I l. AV methodrof producing a long( horizontal fracture'l in Va subterranean elastic formation which is penetrated by a Wellk and which is adjacent a plastic Vformation rcomprising, undercutting saidqplastic formation to produce' a cavity'therein having across-sectional area.sub.

stantially larger than that of said well, isolating said tion,- pausing for a periodof time sufficient to` permit re` laxation of vertical stresses in said plastic formation,

'cavity from fluid communication with said well by running a casing into .said Well and into said elastic formaperforating saidk casingata levelfopposite said elastic formation, and injecting a fracturing fluid through `said Y `well intofsaid elastic formation via the resulting perforations Yunder` suflicient pressure to fracture' said *elasticl formation. -V n j v 2,. YAA 'rnethodof producing along horizontal. fracture inside of vsaid ,A casing isolated from and out of 'uid communication-with said cavity, perforating said casing opposite said elastic formationrso that theinside ofY said casing is in vfluid communication with,v said elastic formationgpausing for a periodof timey suflicient to permit relaaation'of vertical stressespiny saidy plastic formation 75 '3,018,095

7,. A method of producing a'longihorizontalfracture" in a subterranean elastic formation whichis'pene'trated n byf a well and which is superjacent a'plastic formation comprising undercutting said plastic formation to pro' duce a cavity-therein, setting a well casing in .said elastic formation,l isolating fluid in said casing from said cavity, pausing for a period of time sufficient to permit relaxation -of vertical stresses' in said plasticformation perforating said Vcasing atea level opposite said elastic formation and then selectively injecting a fracturing fluid through said well into said. elastic formation via the resulting perforations under sufficient pressure to fracture said elastic formation. f 8.'A method according to claim 7 wherein said Well casing is set and cemented substantially throughhsaidy Y elastic formationr including perforating said casing opposite. said elastic formation'and injecting said fracturing juid through casing perforations Yinto said elastic .formationj Y 9. A `method according to claim- 8f including setting i a bridging plug in said casing at an elevation below the elevation of said. casing perforationsto'isolate the fracturing fluid insaid casing from .saidvcavity and thereby. maintain a relatively lowffluid `pressure in said=fcavity1 during injection of Vsaid fracturing iiuid into saidr elastic formation.` `f i i Y v .y i v' l 10. A method according to claim. 9. Vwhereinlsaid undercutting of said plastic formation. is conducted radiv k ally from said well to produce a cavity substantially colaxial with but largerthan said well. v f

ReferencesiCitedrin the'A le of vthis, patentV UNITEDsrArEs PATENTS 

1. A METHOD OF PRODUCING A LONG HORIZONTAL FRACTURE IN A SUBTERRANEAN ELASTIC FORMATION WHICH IS PENETRATED BY A WELL AND WHICH IS ADJACENT A PLASTIC FORMATION COMPRISING UNDERCUTTING SAID PLASTIC FORMATION TO PRODUCE A CAVITY THEREIN HAVING A CROSS-SECTIONAL AREA SUBSTANTIALLY LARGER THAN THAT OF SAID WELL, ISOLATING SAID CAVITY FROM FLUID COMMUNICATION WITH SAID WELL BY RUNNING A CASING INTO SAID WELL AND INTO SAID ELASTIC FORMATION, PAUSING FOR A PERIOD OF TIME SUFFICIENT TO PERMIT RELAXATION OF VERTICAL STRESSES IN SAID PLASTIC FORMATION, PERFORATING SAID CASING AT A LEVEL OPPOSITE SAID ELASTIC FORMATION, AND INJECTING A FRACTURING FLUID THROUGH SAID WELL INTO SAID ELASTIC FORMATION VIA THE RESULTING PERFORATIONS UNDER SUFFICIENT PRESSURE TO FRACTURE SAID ELASTIC FORMATION. 